Create `__all__` to explicitly export only the important parts

Currently, it generates the following:
```py
# Generated by the protocol buffer compiler. DO NOT EDIT!
# sources: example.proto
# plugin: python-betterproto
from dataclasses import dataclass

import betterproto

@dataclass
class Greeting(betterproto.Message):
"""Greeting represents a message you can tell a user."""

message: str = betterproto.string_field(1)
```

If you try to import from it, you'll get suggestions for importing `dataclass`, `betterproto` and `Greeting`.

Those first two, though, aren't meant to be re-exported there, only `Greeting` is supposed to be importable.

By generating with an `__all__ = ["Greeting"]`, Python would know that the only thing it's supposed to import is `Greeting`:

```py
# Generated by the protocol buffer compiler. DO NOT EDIT!
# sources: example.proto
# plugin: python-betterproto

__all__ = ["Greeting"]

from dataclasses import dataclass

import betterproto

@dataclass
class Greeting(betterproto.Message):
"""Greeting represents a message you can tell a user."""

message: str = betterproto.string_field(1)
```

https://docs.python.org/3/tutorial/modules.html#importing-from-a-package
https://pep8.org/#module-level-dunder-names

L’issue concerne le module Python généré et ses exports publics. Repérez le point d’entrée du générateur qui assemble ce module, puis examinez les tests ou fixtures existants du générateur avant de modifier la sortie. Le travail est considéré comme terminé lorsque les modules générés contiennent un __all__ qui ne liste que les symboles de message et de service prévus, et que les tests couvrent les imports qui en résultent.

Rédigé par le modèle d'indexation à partir du texte de l'issue.